Study On Premixed Combustion Mechanism Of Fuel Gas In Gradually-varied Porous Media

The natural gas is being exploited and utilized rapidly for solving the serious environmental pollution problem due to the bad energy structure in our country. To develop the technique of natural gas combustion with higher efficiency and lower pollutants emission becomes very significant. The technique of fuel gases combustion in inert porous media has many advantages, and becomes an important topic in combustion research field.In this thesis, with high efficiency, stable combustion and low pollutants emission for goal, a idea of gradually-varied porous media (GVPM) was put forward, and the characteristic of the gaseous fuel/air mixture flow and premixed combustion in the GVPM was studied detailedly, based on the study on the influence of the quenching diameter, pore size and thickness of the porous ceramic foam to the characteristic of the natural gas/air premixed combustion in the homogeneous porous media (HPM).Experimental studies are done for the quenching effect of premixed flames in the HPM. It was found that the quenching diameter of porous media is a multi-parametric complicated function, related to the mixture flow speed, the premixed gaseous laminar flame velocity, the pipe flow Reynolds numbers, the coefficient of temperature conductivity, the equivalence ratio and the solid temperature. To make a GVPM with the pores increasing gradually, the quenching diameter range will be expanded and the stability of the flame in it be enhanced.The stability of premixed combustion in HPM was studied experimentally. The flashback, blow-off or extinction equivalence ratio limits were gained for the HPM burner. Similarly, the gas flow speed or the equivalence ratio conditions were studied, which of the flame into the HPM from outside or of the flame away from the HPM. The laws of the flame position and the temperature in the HPM changing with the equivalence ratio or the firing rate have been gained. During the experiments, the phenomena of the flames were observed to be excursion, separate layers or blocks burning. The existence of these phenomena may be one reason of the uniform temperature distribution and low pollutant emission in the porous media..The influence of the pore size, thickness and materials to the characteristics of stable combustion in the HPM were studied experimentally. The influence of the mean pore diameter of the HPM to the temperature distribution in the combustors, pollutants emission, flame speed, burning stability and the pressure drop (resistance loss) were gained. As the case of the HPM with 30 PPI, the influence of the HPM thickness to the characteristics of heat exchange and the resistance of the HPM burners were studied. The studies on the combustion characteristics in the HPM are the base for designing the GVPM and studying the combustion characteristics in the GVPM.A systematic research was carried out on the characteristics of natural gas/air mixture steadily combusting in the GVPM burners. The contents included temperature distribution, pollutant emission, flame speed and pressure loss, as the functions of the equivalence ratio and firing rate, moreover, the gas velocity and equivalence ratio limit for steady combustion in the GVPM burners. There is multiplicity for steady combustion available, which can be indicatedby temperature distribution in the burner. The startup and unsteady combustion characteristics of the GVPM burners were also studied. The experimental results show that the combustion and flow characteristics in porous media burners can be improved by using a suitable GVPM structure. More uniform temperature distributions, better combustion stability, faster flame speed and broader limit range of equivalent ratio were achieved for a GVPM than those of HPM. Meanwhile, lower pollutant emission is realized due to amendatory temperature distribution.The temperature distributions of different characteristics in the GVPM burner can be found under various firing rates. After analyzing the experimental results, the recommended ranges of firing rates for the burners with different GV...